As more states adopt laws allowing medical marijuana, fewer teens see occasional marijuana use as harmful, the largest national survey of youth drug use has found.

Nearly 80% of high school seniors don't consider occasional marijuana use harmful -- the highest rate since 1983-- and record numbers smoke it regularly, according to the annual survey of eighth-, 10th- and 12th-graders made public Wednesday.

More than one in five high school seniors said they smoked marijuana in the month before the survey, and more than a third smoked marijuana during the previous year, according to Monitoring the Future's survey of 45,449 students from 395 public and private schools. The survey has measured drug, alcohol and cigarette use since 1975.

A growing number of state laws that allow marijuana for medical use contributes to teen perceptions that marijuana is not a harmful drug, said Dr. Nora Volkow, director of the National Institutes of Health's National Institute on Drug Abuse, which sponsors the study.

When teens perceive drugs as safe, drug use generally increases, Volkow said. Among eighth-graders, more than 50% don't see the harm of occasional marijuana use while 42% consider occasional use of marijuana harmful -- the lowest rate since the survey began tracking risk perception for this age group in 1991.

A study published this year in the Proceedings of the National Academy of Sciences found that heavy marijuana use beginning as a teen and stretching into adulthood causes an average drop of 8 points in IQ scores.

"That's a very robust indication that (smoking marijuana) may have long-term effects," Volkow said.

The 2012 survey found 6.5% of high school seniors smoke marijuana daily, up from 5.1% five years ago. Almost 23% smoke marijuana regularly. Among 10th graders, 3.5% smoke marijuana daily, the survey found. Marijuana use escalates dramatically after 8th grade, when 1.1% of the students report daily use.

But even most 8th graders don't see the harm of occasional use, the survey found.

"I think that's the bad news in the survey -- the significant increases in the regular use of marijuana," Volkow said. "It's not just the occasional use. You have a very high rate of daily use. That's really a huge number."

Use of other illegal drugs continued to show a steady decline. Past-year use of all illegal drugs except for marijuana is a record lows for all three grades, the survey found.

"These long-term declines in youth drug use in America are proof that positive social change is possible," White House Office of National Drug Control Policy Director Gil Kerlikowske said.

The survey for the first time measured use of an emerging stimulant drug known as "bath salts," and found low use among teenagers. Among 12th graders, 1.3% said they had used the drugs, which can often be purchased on the internet or in drug paraphernalia stores.

Among 12th graders, 11% said they had used synthetic marijuana know as K2 or Spice -- about the same as last year. The federal government recently banned the drugs.